Hello Andrew,
By mixing the output of a differentiator (+6 Db / octave slope for all frequency) with a straight flat gain, you get a response that remains flat with increasing frequency until the differentiator's output dominates -- then its rising response takes over. Both these functions can be combined into a simple 2-tap FIR filter, as shown below:
A FIR differentiator's two coefficients are of opposite polarity, while a straight gain is just one positive coefficient. The attached spreadsheet calculates the combined filter's coefficients given the sample rate and the desired corner frequency.
Best regards,
Bob